1. Field of the Invention
The present invention relates to a fixing device for thermally fixing an unfixed image on a recording medium, and a device and a method for replenishing a releasing agent for use in the fixing device.
2. Description of the Related Art
An image forming apparatus such as an electrophotographic printer, copying machine, is normally equipped with a fixing device that fixes an unfixed toner image on a sheet as recording medium based on the pressure-thermal fixing process.
In such a fixing device, the sheet surface that retains the toner comes in direct contact with the surface of the fixing roller. As a result, it often causes a phenomenon called "offset" because unfixed toner on the sheet smears the fixing roller.
The offset phenomenon has been dealt with coating the surface of the fixing roller by means of a releasing agent coating mechanism with a releasing agent that helps to release the toner from the fixing roller smoothly, and maintaining the surface of the fixing roller clean using a cleaning roller and the like. Since silicon oil is normally used as the releasing agent, the releasing agent will be also called the "oil" in the following discussion for the sake of convenience.
The function of the releasing agent coating mechanism is to apply the oil to the surface of the fixing roller adequately as the oil is taken away by the sheet from the fixing roller surface each time the sheet passes the roller, or when the printing paper feed occurs. Consequently, the releasing agent coating mechanism is also referred to as the oil replenishing mechanism.
An oil replenishing mechanism of the prior art is disclosed in the Japanese Patent Unexamined Publication, JP-A-60-151680. The oil replenishing mechanism includes a cylindrical part, which has multiple small holes and holds the oil internally, a heat resistant porous elastic part that covers the outside of the cylindrical part, and an oil replenishing roller coated with a silicon varnish thin film that covers the outside surface of the heat resistant elastic part.
However, such an oil replenishing mechanism has a shortcoming that there is a limit to the amount of oil it can hold inside the cylindrical part, so that it has only a short life as an oil replenishing device.
Another Japanese Patent Unexamined Publication, No. P-A-05-265346, discloses an oil replenishing mechanism that improves the shortcoming described above. The oil replenishing mechanism comprises: an oil coating roller that holds the oil inside, is in contact with the fixing roller under a pressure to be driven by the latter, and coats the fixing roller surface with the oil seeping out from the inside of the coating roller; and an oil replenishing device that replenishes the oil inside the coating roller from an external source; where the coating roller and the replenishing device are combined into a single sealed unit.
However, it is difficult to make the oil exist in a stable manner inside the coating roller of such an oil replenishing mechanism. Also, the coating roller has another shortcoming that its coating action is sometimes not uniform because it makes starts and stops repeatedly. In particular, the oil tends to gather at the lower portion of the coating roller due to the gravity when the coating roller is at still.
If the fixing roller is coated under such a condition, the oil seeping amount becomes uneven and hence the coating of the oil becomes uneven around the periphery of the fixing roller. Consequently, the offset phenomenon cannot be completely eliminated with such devices or mechanisms. The offset phenomenon is more conspicuously on a device or apparatus that handles color images, causes uneven luster and image noises.